It’s not very common with spam here but it was one message a few moments ago. I just changed the title to begin with “spam” but I couldn’t find any other way to mark it as spam. Am I missing something?

To the left of the Reply button, click on the … button to bring up more actions. Then there is a flag button just to the left of that for “privately flag this post for attention.” It brings up a pop-up dialog and you can complain. I already did it for the message you referred to.